The best data currently available suggest that significant population doses are being sustained via the action of a little-studied dose mechanism - the introduction of radon-222 into indoor atmospheres by the domestic use of ground waters naturally high in radon. The major health outcome associated with exposure to radon and its progeny is carcinoma of the bronchial epithelium. Extrapolations based on present information show that if the linear non-threshold hypothesis holds, then increased mortality due to lung cancer attributable to this factor should be onservable in the U. S. population. However, much work must be done before an meaningful epidemiological investigation can be undertaken. Only a small percentage of the public water supplies in the U. S. have been analyzed for radon, and the relationship between radon concentrations in water and ultimate radiation dose to the bronchial epithelium is quite complicated and not entirely understood. This project seeks to increase our understanding of the entire exposure mechanism so that meaningful population dose estimates can be made, and to define appropriate exposed and control populations for future epidemiological work. The water supplies of communities selected for physical and demographic characteristics will be surveyed for radon, and experiments will be performed to refine our understanding of factors modifying the ultimate dose to the target cells. These findings will be used to predict excess mortality in the exposed populations. If these figures are found to be as high as current data suggests, then this exposure mechanism should provide an unprecedented opportunity to test the linear non-threshold hypothesis at low dose levels in a human population.